Process for producing sintered silicon carbide ceramic body

ABSTRACT

Pressureless sintering of silicon carbide to produce ceramic bodies having 75% and greater theoretical densities, can be accomplished by firing shaped bodies, containing finely divided silicon carbide, boron source such as boron carbide, carbon source such as phenolic resin and a temporary binder, at a sintering temperature of from about 1900° C to about 2500° C.

This is a division of copending application Ser. No. 584,226, filed June5, 1975.

BACKGROUN OF THE INVENTION

The chemical and physical properties of silicon carbide made it anexcellent material for high temperature structural applications. Theseproperties include good oxidation resistance and corrosion behavior,good heat transfer coefficients, low expansion coefficient, high thermalshock resistance and high strength at elevated temperature. It is inparticular desirable to produce silicon carbide bodies having highdensity and suitable for engineering material uses, such as for examplehigh temperature gas turbine applications. Silicon carbide is apreferred material for such use, because it can withstand greatertemperature differential than convention materials, and can thereforelead to greater efficiency in the transformation of energy.

Methods of producing high density silicon carbide bodies have heretoforeincluded reaction bonding (also known as reaction sintering), chemicalvapor deposition and hot pressing. Reaction sintering involves the useof silicon impregnants to upgrade the density of the silicon carbide andis useful for many applications, but is undesirable where excess siliconexuding from the silicon carbide body would be detrimental. Siliconcarbide deposition is impractical for producing complex shapes, and hotpressing (the production of high density silicon carbide bodies bysimultaneous application of heat and pressure) is impractical for someshapes, since the pressure required during the hot pressing operationdeforms the silicon carbide body and requires that only relativelysimple shapes can be produced by this method.

It is, therefore, an object of this invention to produce a sinteredceramic body having a high proportion of silicon carbide, and a high(greater than 75% theoretical) density. It is a further object of thisinvention to produce such a body which does not require the use ofexpensive and hard to obtain finely divided "beta" (cubic) siliconcarbide, which has heretofor been reguarded as a highly preferred rawmaterial for such ceramic body, due to the previously found difficultiesin obtaining sintering of mixtures containing alpha (non-cubic) siliconcarbide material.

Subsidiary objects of this invention are the provision of a raw batchand a process for the production of such sintered ceramic bodycontaining high proportion of silicon carbide and high density.

SUMMARY OF THE INVENTION

According to the first aspect of the present invention, there isprovided a sintered ceramic body consisting essentially of from about 91to about 99.35% by weight silicon carbide, from about 0.5 to about 5.0%carbonized organic material, from about 0.15 to about 3.0% boron, and upto about 1.0% additional carbon; and having a density of at least about2.40 g/cc. According to a second aspect of the present invention, thereis provided a raw batch for producing a sintered ceramic body,comprising from about 91 to about 99.35 parts by weight silicon carbidehaving a surface area of from about 1 to about 100 m² l/g; (from about0.67 to about 20 parts by weight of a carbonizable, organic solventsoluble, organic material having a char yield of from about 25 to about75% by weight; from about 0.15 to about 5 parts by weight of a boronsource containing from about 0.15 to about 3.0 parts by weight boron;and from about 5 to about 15 parts by weight of temporary binder.According to a further aspect of this invention, there is provided aprocess for producing a sintered ceramic body, comprising the steps ofmixing together the ingredients of the above-described raw batch; addingto the raw batch from about 25 to about 100% by weight of the raw batchof an organic solvent in which the carbonizable, organic solventsoluble, organic material is soluble; stirring the raw batch and organicsolvent in such a way as to disperse the carbonizable, organic solventsoluble, organic material about the silicon carbide of the raw batch;drying the stirred mixture in such a way as to evaporate the organicsolvent from the mixture; shaping the dried mixture in such a way as toproduce a shaped body having a density of at least about 1.60 g/cc;curing the temporary binder within the shaped body; and firing theshaped body for such time, such temperature and in such environment asto produce a density of at least about 2.40 g/cc.

BRIEF DESCRIPTION OF THE DRAWING

The single FIGURE of drawing is a flow chart which shows the admixtureof four raw materials to form a raw batch; the addition of solvent andsubsequent processing step to disperse the carbon source about thesilicon carbide of the raw batch; and the subsequent processing steps toproduce the sintered ceramic body of the present invention.

DETAILED DESCRIPTION

The theoretical density of silicon carbide is 3.21 g/cc. The preferredminimum density of about 2.40 g/cc for sintered ceramic bodies accordingto the present invention corresponds, therefore, to about 75% oftheoretical density, a density factor which has been previouslydifficult if not impossible to obtain without the use of beta siliconcarbide powders. The beta powders, however, are more difficult toobtain, and are more expensive, than the more common alpha (non-cubic)crystalline forms, or the amorphous (non-crystalline) form. It has beenfound that the process of the present invention is essentially immune tochanges in crystallinity of the silicon carbide powder startingmaterial, unlike various pressureless sintering and hot pressingprocesses of the prior art. Indeed, according to the present invention,sintered ceramic bodies wherein the silicon carbide consists essentiallyof alpha, non-cubic silicon carbide and which have densities in excessof about 2.90 g/cc (corresponding to 90% theoretical density of siliconcarbide) are easily obtained. It is not by any means, however, necessarythat the silicon carbide consist essentially of alpha non-cubic siliconcarbide, although this is preferred. It has indeed been shown to bepossible, in accordance with the present invention, to produce sinteredceramic bodies in which the silicon carbide is predominantly (more than50%) alpha, non-cubic silicon carbide. Indeed, various mixtures of alphaand beta crystalline material, containing at least 5 % alpha, non-cubicsilicon carbide, have been shown to be operable for use in the presentinvention. Various amounts of amorphous non-crystalline silicon carbidepowders can also be used. The process of the present invention does notproduce any substantial amount of phase transformation, so that thecrystalline habit of the silicon carbide in the starting material willbe in essentially the same proportions as the crystalline habit of thefinished sintered ceramic body.

As noted above, the minimum preferred density for sintered ceramicbodies according to the present invention is about 2.40 g/cc (75%theoretical). Such bodies can be used "as is" for some applications, orthey may be machined at the more manageable density of 2.40 orthereabouts, and later subjected to further firing to produce furtherdensification of the sintered ceramic bodies. For use as turbine bladesand many other high temperature applications, it is preferred that thedensity be at least about 3.05 g/cc (95% of theoretical). Such densityis readily obtainable in accordance with the present invention.

The preferred composition of the sintered ceramic body in accordancewith the present invention consists essentially of from about 91 toabout 99.35% by weight silicon carbide, from about 0.5 to about 5.0%carbonized organic material, from about 0.15 to about 3.0% boron, and upto about 1.0% additional carbon. All percentages herein are by weight,unless otherwise specified. Within this broad composition, it ispreferred that the sintered ceramic body contain from about 0.5 to about4% carbonized organic material, from about 0.18 to about 0.36% boron,about 0.05 to about 0.10% additional carbon, with the balance of thecomposition being silicon carbide. In particular, it is preferred thatthe sintered ceramic body contain about 2% carbonized organic material,about 0.36% boron, and about 0.1% additional carbon. This is thecomposition which results from practicing the process of the presentinvention in accordance with the best mode now contemplated. Within thebroad range of ingredients specified, the density obtained appears tohave relatively little correlation to ingredients, but is rather afunction of firing conditions, particularly the temperature and time offiring.

The sintered ceramic body should contain from about 0.15 to about 3.0%boron. In so specifying the boron content, it is not intended to limitthe form in which the boron may be present (i.e., whether elementalboron or combined in boron compounds such as boron carbide). Indeed, itis believed that in most cases the boron will be present as a boroncarbide material in the finished sintered ceramic body. The "additionalcarbon" broadly specified as being present in an amount up to about 1.0%is thus an optional ingredient of the finished sintered ceramic body, asthe amount of such additional carbon (i.e., carbon other than that ofthe silicon carbide or that which is present as carbonized organicmaterial) will depend on the carbon associated with the boron which waspresent in the raw batch mixture from which the sintered ceramic bodywas made.

The amount of carbon which is present as carbonized organic materialwill depend on the amount of such organic material which was added tothe raw batch, and the char yield (carbon content) of the organicmaterial.

The raw batch for producing the sintered ceramic body in accordance withthe present invention comprises from about 91 to about 99.35 parts byweight silicon carbide having a surface area of from about 1 to about100 m² /g. Such silicon carbide powders are usually less than 20 micronsin particle size, more particularly less than 10 microns in particlesize; and in particular sub-micron size particles are generallypreferred. It is difficult, however, to obtain accurate particle sizedistributions for silicon carbide powders having size much less than 1micron in particle size, and the surface area of the silicon carbideparticle is the more relevant consideration in determining suitablematerial. Accordingly, the preferred silicon carbide particles for usein the process of the present invention to produce the sintered ceramicbodies of the present invention are specified as having from about 1 toabout 100 m² /g surface area. Within this range, it is more preferredthat the surface area of the silicon carbide particles range betweenabout 5 and 50 m² /g; and within this range, surface areas between about7 and about 15 m² /g have been found to be easily obtainable and quiteuseful for producing sintered ceramic bodies of the present invention.

The silicon carbide starting material can be obtained from any of avariety of sources. Vapor phase reacted material is produced in fineparticle size, and can be used if desired. Larger material can be ballmilled until sufficient amounts of fine silicon carbide are obtained,and the proper size of silicon carbide can be selected from the ballmilled product by conventional means, such as water sedimentation.

The crystalline habit of the silicon carbide starting material in theraw batch is essentially non-critical. Alpha, non-cubic silicon carbideis more readily available than beta silicon carbide, and therefore thepreferred starting material contains silicon carbide which consistsessentially of alpha, non-cubic crystalline silicon carbide. It is quiteacceptable, however, to use silicon carbide which has been made by aprocess which produces mixtures of alpha and beta silicon carbide, andthe next most preferred starting material is silicon carbide which ispredominantly alpha, non-cubic crystalline silicon carbide. It has alsobeen shown to be operable to use raw batches wherein the silicon carbidecomprises at least 5% alpha, non-cubic crystalline silicon carbide, andamorphous silicon carbide can also be used. It is even possible to usehigh purity beta silicon carbide starting material, but such material isnot preferred because of the high expense of obtaining high purity betasilicon carbide powders.

In any event, it is preferred that the silicon carbide material shallhave been treated with acid (such as hydrofluoric and/or nitric acids,particularly mixtures of hydrofluoric and nitric acids) to removeextraneous materials which may interfere with the sintering operation.

One of the more important features of the raw batch of the presentinvention is the carbonizable, organic solvent soluble, organicmaterial. It has been found desirable that this material be organic andorganic solvent soluble in order that it be easily dispersable about thesilicon carbide particle of the raw batch, in order to provide anintimate availability of carbonized organic material upon firing of theshaped body produced from the raw batch. It has been found desirablethat the sintered ceramic body contain from about 0.5 to about 5.0% ofcarbonized organic material, with the result that if the carbonizable,organic solvent soluble, organic material has a char yield of from about25 to about 75% by weight, as is preferred, there should be present fromabout 0.67 to about 20 parts by weight of carbonizable, organic solventsoluble, organic material in the raw batch. Within the range of fromabout 25 to about 75% by weight char yield, it is preferred that theorganic material have from about 33 to about 50% by weight, moreparticularly from about 40 to about 45% by weight, char yield. If thechar yield is between about 33 and about 50% by weight, the amount ofcarbonizable, organic solvent soluble, organic material should rangebetween about 1 and 12% by weight to produce the preferred amount ofcarbonized organic material from about 0.5 to about 4.0% by weight inthe finished sintered ceramic body. The most preferred amount ofcarbonized organic material in the sintered ceramic body is believed tobe about 2% by weight, so that the optimum raw batch should containabout 5% by weight of an organic solvent soluble, organic materialhaving a char yield between about 40 and 45% by weight. Particularlypreferred carbonizable, organic solvent soluble, organic materials arephenolic resin and coal tar pitch, which have char yields of from about40 to about 42% and on the order of 60%, respectively. As between thephenolic resin and coal tar pitch, the phenolic resin is more definitelypreferred, and particularly a B-stage resole phenolic resin has beenfound to be particularly useful in the present invention.

The boron can be added to the raw batch as either elemental boron or asboron carbide. Boron carbide is essentially a non-stoichiometricmaterial, and various boron carbide materials having a molar ratio ofboron to carbide between 8:1 and 2:1 have been reported. It is ingeneral preferred to use boron carbide as the boron source, and inparticularly boron carbide which is so-called "solid state reacted boroncarbide" with a molar ratio of boron to carbon between about 3.5:1 and4.1:1. Such boron carbide can be produced in accordance with the processof U.S. Pat. No. 3,379,647, P. A. Smudski. The process of the aboveSmudski patent is found to produce boron carbides having such a molarratio, and such a molar ratio is preferred because with the higher boronto carbon ratio, the boron carbide either takes carbon or gives boron tothe surrounding chemical species, which is desirable in the presentinstance as it promotes the desired densification during the firing stepof the process of the present invention. Boron carbide materials havinggreater ratios of boron to carbide are even more chemically active thanthe material having a ratio of about 4.1:1 to about 3.5:1, but suchmaterials are relatively less available and more expensive, andtherefore are not preferred for that reason.

The amount of boron source to be added to the raw batch depends on theboron content of the boron source and the amount of boron to be presentin the final sintered ceramic body. The sintered ceramic body shouldcontain from about 0.15 to about 3.0% boron, and in particular fromabout 0.18 to about 0.36% boron is present in the most successfullydensified bodies produced in accordance with the present invention.0.36% is the optimum boron content of the sintered ceramic body. Theamount of boron source should thus be chosen accordingly. Thus, if theboron source is elemental boron, it should be present in the raw batchfrom about 0.18 to about 0.36 parts by weight to yield a sinteredceramic body having from about 0.18 to about 0.36% by weight boron. Forthe preferred solid state reacted boron carbide with a molar ratio ofboron to carbon between about 3.5:1 and about 4.1:1, the boron carbideshould be present in an amount from about 0.23 to about 0.46 parts byweight to produce such an amount of boron in the finished sinteredceramic body.

In any event, the boron source can be crystalline or non-crystalline,and preferably is particulate and of a size less than 30 microns. Withinthis limitation, it is preferred that the boron source be of a sizeranging from about 0.1 to about 10 microns.

The temporary binder is preferably polyvinyl alcohol having associatedtherewith from about 5 to about 15 parts by weight water, per part ofpolyvinyl alcohol, as a temporary binder vehicle. In particular, it ispreferred to use 10 parts by weight polyvinyl alcohol plus about 90parts by weight water as a temporary binder vehicle. In addition topolyvinyl alcohol, however, other temporary binders can be used, such ascoal tar pitch, long chain fatty material (for example "CARBOWAX" wax),metallic stearates such as aluminum stearates and zinc stearates,sugars, starches, alginates, and polymethyl phenylene. Many of thesematerials are, of course, capable as functioning as the carbonizable,organic solvent soluble, organic material which is added in sufficientquantity to yield the appropriate amount of carbonized organic materialin the finished sintered ceramic body. A single material can thus servetwo functions in the raw batch.

The process for producing the sintered ceramic body according to thepresent invention preferably begins with the mixing together of theingredients of the raw batch, namely from about 91 to about 99.35 partsby weight silicon carbide; from about 0.67 to about 20 parts by weightof the carbonizable organic material; from about 0.15 to about 5% byweight of the boron source; and from about 5 to about 15 parts by weightof temporary binder. If the temporary binder is polyvinyl alcoholincluding a quantity of water as temporary binder vehicle, this firstmixing step preferably includes stirring the powdered materials (siliconcarbide, organic material and boron source) together with the temporarybinder and temporary binder vehicle, prior to adding an organic solventin which the organic material is soluble. In any event, after theorganic solvent is added, the raw batch and organic solvent should bestirred in such a way as to disperse the carbonizable, organic solventsoluble, organic material about the silicon carbide of the raw batch,suitably for at least about 5 minutes, and preferably about 15 minutes.After the raw batch and organic solvent have been stirred so as todisperse the organic material about the silicon carbide, the stirredmixture is dried by any suitable technique, such as passing a quantityof drying gas near the stirred mixture, or by spray-drying the mixture.Following this drying step, the dried mixture is shaped in such a way asto produce a shaped body preferably having a density of at least about1.60 g/cc. This shaping can be accomplished by any of a variety oftechniques which are in themselves known, for example by extrusion,injection molding, transfer molding, casting, cold pressing, isostaticpressing, or by compression. If compression is used, preferred pressuresare between about 4,000 and about 100,000 psi, with between about 16,000and about 20,000 psi being preferred. If the temporary binder ispolyvinyl alcohol, the next step of curing the temporary binder can bepreferably accomplished by heating the shaped body at a temperatureabout 90° to about 100° C for about 1 to about 2 hours. The shaped bodyis then fired to accomplish the densification necessary to produce thesintered ceramic body of the invention. Firing takes from about 20 toabout 60 minutes at temperatures of from about 1900° to about 2500° C.Lower temperatures are in general inoperable, and higher temperaturesmay cause sublimation of the silicon carbide material. The firing stepcan be carried out in an conventional tube furnace wherein the shapedbody is passed through the hot zone of the tube furnace to having aresidence time at the desired temperature and for the desired time.Details of such tube furnaces are known in the prior art, and aredescribed for example in P. A. Smudski, U.S. Pat. No. 3,689,220. Thefiring step accomplishes a "pressureless sintering," referred to hereinfor simplicity merely as "sintering." By "sintering" or "pressurelesssintering" it is meant that no mechanical pressure is applied to theobject being fired or sintered to enhance the reaction. Instead, theobject being sintered is surrounded, usually in an inert container suchas a graphite crucible, in up to about 1 atmosphere of pressure of aninert gas, a reducing gas, a vacuum, or nitrogen. Reducing gases includehydrogen, carbon dioxide and carbon monoxide; inert gases include argon,helium, and neon. The gases in which the sintering operation can becarried out thus include argon, carbon dioxide, carbon monoxide, helium,hydrogen, neon and nitrogen. Although nitrogen enters into reaction in aminor degree with the silicon carbide raw material, it does so insufficiently minor degree that the composition of the sintered ceramicbody is not noticeably changed. The use of nitrogen, however, does raisethe necessary sintering temperature about 200° C, so that if nitrogen isthe surrounding atmosphere, the preferred sintering temperature is fromabout 2260° to about 2300° C. In the other gases, particularly inertgases such as argon, helium or neon, the preferred sintering temperatureis from about 2060° to about 2100° C.

The firing can also be carried out under vacuum, i.e., without anysurrounding atmosphere. By "vacuum" is meant a practical vacuum, i.e.,1.0 mmHg or less.

The invention will now be illustrated with several examples.

EXAMPLE 1

The raw materials mixed together for the raw batch of this example weresub-micron alpha silicon carbide, solid state reacted boron carbide(B:C=4.08:1), B-stage resole phenolic resin identified by VarcumChemical Company as Resin 8121, and polyvinyl alcohol in water. Thesilicon carbide has a surface area between about 7 and 15 m² /g; 9.764grams were used. Particulate boron carbide having a size less than 10microns, containing 0.036 grams boron, was used. The silicon carbide andboron carbide and 0.5883 grams phenolic resin were placed in a 4-ouncebottle. One gram of a 10% polyvinyl alcohol-water solution were addedand the ingredients were mixed together for 15 minutes. Twentymilliliters of acetone, a solvent for the organic phenolic resin, wasadded and the mixture again stirred for 15 additional minutes. Nitrogenwas passed into the container gently so as to evaporate the acetone andwater from the mixture, leaving a powder which was dry to the touch. Themix was occasionally stirred during evaporation, which was continued forone-half hour. When the mix reached a putty-like consistency it wasstirred continuously until the mix began to break up into fineparticles. When there was only a faint trace of acetone smell and thematerial was dry to the touch, the mix was judged ready for shaping intoa body for curing and firing. A portion of the powder was compressed at16,000 psi. After pressing the temporary binder was cured at 100° C fora time between 1 and 2 hours. After curing the density was found to be1.78 g/cc. The cured piece was placed on a graphite setter and thenplaced in a closed graphite crucible. The crucible was fed into a tubefurnace having a hot zone maintained at 2080° C, at a rate of about23/4" per minute, so that it required about 20 minutes to traverse the54 inches to the hot zone. Argon was passed through the tube furnaceduring this time at about 1 atmosphere of pressure (zero gaugepressure). The cured body was held in the hot zone of 2080° C for 45minutes, and held for about 20 minutes in a cooling chamber to avoidthermal shock. After the body had cooled, the density was again checkedand the sintered ceramic body was found to have a density of 2.75 g/cc,about 86% of the theoretical 3.21 g/cc density.

EXAMPLES 2-16

Example 1 was repeated with various silicon carbide samples. The resultsfor Examples 1-16 are tabulated in Table 1.

EXAMPLES 17-20 CONTROL EXPERIMENTS 1 AND 2

Example 1 was again repeated, varying the atmospheres (or lack ofatmosphere) and firing temperature. Results are set forth in Table 2.

EXAMPLES 21-35 CONTROL EXPERIMENTS 3 AND 4

Example 1 was again repeated in each case changing one variable. Theresults are set forth in Table 3.

                                      TABLE 1                                     __________________________________________________________________________                                  APPROXIMATE                                                      "GREEN"                                                                             SINTERED                                                                             SINTERED PERCENT                                       SURFACE AREA                                                                            DENSITY                                                                             DENSITY                                                                              THEORETICAL                                     EXAMPLE                                                                              (m.sup.2 /g)                                                                            (g/cc)                                                                              (g/cc) DENSITY                                         __________________________________________________________________________    1      13.2-14   1.78  2.75   86                                              2      13.2-14   1.72  2.86   89                                              3      13.2-14   1.71  2.82   88                                              4      13.5      1.71  2.72   85                                              5      13.5      1.72  2.71   85                                              6      13.5      1.71  2.73   85                                              7      13.2      1.70  3.07   96                                              8      13.2      1.70  3.05   95                                              9      13.5      1.69  2.62   82                                              10     9.5       1.84  3.09   96                                              11     9.5       1.81  3.05   95                                              12     9.5       1.87  3.08   96                                              13     9.5       1.80  2.98   93                                              14     13.5      1.82  2.90   90                                              15     7         1.83  2.59   81                                              16     9.5       1.80  3.00   93                                              __________________________________________________________________________

                                      TABLE 2                                     __________________________________________________________________________                                                      APPROXIMATE                                                     "GREEN"                                                                             SINTERED                                                                              SINTERED PERCENT                            TEMPERATURE                                                                             SURFACE AREA                                                                            DENSITY                                                                             DENSITY THEORETICAL                 EXAMPLE                                                                              ATMOSPHERE                                                                             ° C                                                                              (m.sup.2 /g)                                                                            (g/cc)                                                                              (g/cc)  DENSITY                     __________________________________________________________________________    17     Nitrogen 2280      13.2      1.67  3.08    96                          18     Nitrogen 2280      9.5       1.79  2.50    78                          19     Vacuum   2080      9.5       1.79  2.76    86                          20     Vacuum   1900      9.5       1.80  2.40    75                          CONTROL 1                                                                            Nitrogen 2080      9.5       1.85  1.84    57                          CONTROL 2                                                                            Nitrogen 2080      9.5       1.78  1.84    57                          __________________________________________________________________________

                                      TABLE 3                                     __________________________________________________________________________                                                         APPROXIMATE                                                                   SINTERED                                                        "GREEN"                                                                              SINTERED                                                                             PERCENT                          SURFACE                        DENSITY,                                                                             DENSITY,                                                                             THEORETICAL              EXAMPLE AREA, m.sup.2 /g                                                                      VARIED ITEM            g/cc   g/cc   DENSITY                  __________________________________________________________________________    21      9.5     B source is arc furnaced 1000 grit B.sub.4 C                                                         1.83   3.06   95                       22      9.5     B source is amorphous elemental boron                                                                1.84   2.94   92                       23      9.5     B source decreased to yield 0.18% boron                                                              1.87   2.99   93                       24      9.5     B source increased to yield 0.72% boron                                                              1.89   2.84   88                       25      9.5     B source increased to yield 3% carbon                                                                1.77   3.05   95                       26      7       C source increased to yield 3% carbon                                                                1.76   2.48   77                       27      9.5     C source increased to yield 4% carbon                                                                1.95   2.97   93                       28      9.5     C source is coal tar pitch                                                                           1.78   2.44   76                       29      9.5     Firing time is 30 minutes                                                                            1.90   2.86   89                       30      9.5     Firing time is 20 minutes                                                                            1.91   2.81   88                       31      9.5*    SiC is 5% beta, 95% alpha                                                                            1.86   3.07   96                       32      9.5*    SiC is 25% beta, 75% alpha                                                                           1.86   3.07   96                       33      9.5*    SiC is 50% beta, 50% alpha                                                                           1.86   3.05   95                       34      9.5*    SiC is 75% beta, 25% alpha                                                                           1.87   3.03   94                       35      9.5*    SiC is 95% beta, 5% alpha                                                                            1.87   2.94   92                       CONTROL 3                                                                             9.5     Boron source absent    1.86   2.11   66                       CONTROL 4                                                                             9.5     Carbon source absent   1.72   2.11   66                       __________________________________________________________________________     *Value is for alpha material; beta is about 6 m.sup.2 /g.                

We claim:
 1. A process for producing a sintered ceramic body, comprisingthe steps of:(a) mixing together a raw batch comprising:(i) from about91 to about 99.35 parts by weight alpha silicon carbide and having asurface area of from about 1 to about 100 m² /g; (ii) from about 0.67 toabout 20 parts by weight of a carbonizable, organic solvent soluble,organic material having a char yield of from about 25 to about 75% byweight; (iii) from about 0.15 to about 5 parts by weight of a boronsource containing from about 0.15 to about 3.0 parts by weight boron;and (iv) from about 5 to about 15 parts by weight of temporary binder;(b) adding to the raw batch from about 25 to about 100% by weight of theraw batch of an organic solvent in which the carbonizable, organicsolvent soluble, organic material is soluble; (c) stirring the raw batchand organic solvent in such a way as to disperse the carbonizable,organic solvent soluble, organic material about the silicon carbide ofthe raw batch; (d) drying the stirred mixture in such a way as toevaporate the organic solvent from the mixture; (e) shaping the driedmixture in such a way as to produce a shaped body having a density of toleast about 1.60 g/cc; (f) curing the temporary binder within the shapedbody; and (g) firing the shaped body for such time, at such temperatureand in such environment as to produce a density of at least about 2.40g/cc.
 2. A process according to claim 1, wherein the stirring continuesfor at least about 5 minutes.
 3. A process according to claim 2, whereinthe stirring continues about 15 minutes.
 4. A process according to claim1, wherein the shaping is by extrusion.
 5. A process according to claim1, wherein the shaping is by compression at a pressure between about4,000 and about 100,000 psi.
 6. A process according to claim 5, whereinthe shaping is by compression at a pressure between about 16,000 andabout 20,000 psi.
 7. A process according to claim 1, wherein thetemporary binder is polyvinyl alcohol and the curing is accomplished byheating the shaped body at a temperature of about 90° to about 100° Cfor about 1 to about 2 hours.
 8. A process according to claim 1, whereinthe shaped body is fired for from about 20 to about 60 minutes; at atemperature of from about 1900° to about 2500° C; and in a vacuum.
 9. Aprocess according to claim 1, wherein the shaped body is fired for fromabout 20 to about 60 minutes; at a temperature of from about 1900° toabout 2500° C; and in up to about 1 atmosphere of pressure of a gasselected from the group consisting of argon, carbon dioxide, carbonmonoxide, helium, hydrogen, neon, nitrogen and mixtures thereof.
 10. Aprocess according to claim 9, wherein the gas is approximately 1atmosphere of the member selected from the group consisting of argon,helium and neon; and the temperature is from about 2060° to about 2100°C.
 11. A process according to claim 9, wherein the gas is about 1atmosphere of nitrogen, and the temperature is from about 2260° to about2300° C.
 12. A process according to claim 1, wherein the shaping is byinjection molding.
 13. A process according to claim 1, wherein theshaping is by transfer molding.
 14. A process according to claim 1,wherein the shaping is by casting.
 15. A process according to claim 1,wherein the shaping is by cold pressing.
 16. A process according toclaim 1, wherein the shaping is by isostatic pressing.
 17. A process forproducing a sintered ceramic body, comprising the steps of:(a) mixingtogether a raw batch comprising:(i) from about 91 to about 99.35 partsby weight amorphous silicon carbide having a surface area of from about1 to about 100 m² /g; (ii) from about 0.67 to about 20 parts by weightof a carbonizable, organic solvent soluble, organic material having achar yield of from about 25 to about 75% by weight; (iii) from about0.15 to about 5 parts by weight of a boron source containing from about0.15 to about 3.0 parts by weight boron; and (iv) from about 5 to about15 parts by weight of temporary binder; (b) adding to the raw batch fromabout 25 to about 100% by weight of the raw batch of an organic solventin which the carbonizable, organic solvent soluble, organic material issoluble; (c) stirring the raw batch and organic solvent in such a way asto disperse the carbonizable, organic solvent soluble, organic materialabout the silicon carbide of the raw batch; (d) drying the stirredmixture in such a way as to evaporate the organic solvent from themixture; (e) shaping the dried mixture in such a way as to produce ashaped body having a density of at least about 1.60 g/cc; (f) curing thetemporary binder within the shaped body; and (g) firing the shaped bodyfor such time, at such temperature and in such environment as to producea density of at least about 2.40 g/cc.